冷喷涂铝合金涂层的显微组织与局部腐蚀的关联性研究

Cold spray is the latest technical development among thermal spray processes. Unlike the other thermal spray techniques, its process temperature is much below the melting point of feedstock. In cold spray process, a highly pressurized gas is preheated and then expanded in a converging—diverging type nozzle. A fluidized powder feedstock is fed into or upstream of the nozzle and accelerated by the gas stream and impacts substrate with high velocities..There is great interest as cold spray has the advantage of producing coatings with very low oxygen content and low porosity. As general thermal sprayed coatings, a cold sprayed coating is applied to meet specific requirements for the surface properties. For the majority of all thermal spray applications, these requirements are either wear resistance or corrosion resistance. Literatures have suggested that the use of cold sprayed coatings for corrosion protection is relatively modest compared to the use of thermal sprayed coatings. It is argued that for many applications a cold sprayed coating may well be the most economical solution, in consideration of the characteristic features of cold sprayed coatings. However, there is little data on the corrosion performance of cold sprayed coatings and among potential applications few have been tested for corrosion resistance. An additional difficulty of study on corrosion behavior stems from the morphology of cold sprayed coatings, even the oxygen content and porosity of deposits is extremely low. The properties of a cold sprayed coating could differ from its compositional bulk equivalent because of particles extreme plastic deformation. Moreover, mechanistic studies are very few..Meanwhile there are many key problems which have not been settled. For example, the relationship between coating microstructure and corrosion behavior, the difference of corrosion behavior of coatings and bulk materials caused by particles plastic deformation, the galvanic effect between coating and substrate, etc. .This project chooses aluminum alloy coatings as main research subject. Various characterization methods, such as Scanning Electron Microscope (SEM), X-Ray Tomography and Electron Backscattered Diffraction (EBSD) etc. will be used to study microstructure (porosity of coating, interface characterization between coating and substrate, and crystal orientation and grain size varied by plastic deformation etc.) of single splat and cold sprayed coatings. Electrochemical techniques including Open Circuit Potential Monitoring, Polarization Scanning, Electrochemical Impedance Spectroscopy, Galvanic corrosion, Scanning Vibrating Electrode Technique (SVET) and Local Electrochemical Impedance Spectroscopy (LEIS), will be used to study corrosion behavior，especially local corrosion of cold sprayed aluminum alloys. The relationship between microstructure and corrosion behavior will be focused to identify key characteristics governing the corrosion resistance. Attention will also be paid to the effect of plastic deformation of particles and the galvanic effect between coating and substrate on corrosion protection in order to understand the corrosion mechanism of cold sprayed coatings.

冷喷涂技术是基于空气动力学的新型涂层制备方法。因其较低的气体温度，喷涂材料的化学成分及结构特征可以保留至涂层中，使其在制备纳米材料和非晶材料等领域体现出巨大潜力。同时，致密、低缺陷的显微组织特点也使冷喷涂涂层在腐蚀防护方面显示出明显优势,然而冷喷涂涂层局部腐蚀的机理尚未明确。申请者已成功制备多种冷喷涂涂层，发现在不同的基体材料上铝合金涂层的腐蚀失效方式不同，本项目将从单一沉积粒子（Single Splat）入手，采用各种材料显微组织表征手段以及微区电化学测试方法，研究冷喷涂铝合金颗粒大塑性变形、涂层显微组织、涂层与基体界面结合特征以及界面区涂层与基体的伽伐尼作用，并分析上述因素对冷喷涂涂层局部腐蚀和涂层对基体腐蚀防护性能的影响，阐明塑性变形-涂层显微组织-界面行为-腐蚀机理之间的本质联系，对冷喷涂技术的发展和冷喷涂涂层在我国工业的广泛应用具有重要的理论意义与实际价值。

冷喷涂技术是一种可以用于制备致密防腐蚀涂层的新型涂层制备方法。由于制备工艺不同导致微观结构不同，从而使得冷喷涂涂层的腐蚀行为与相应的块体材料不同。本项目采用不同方法研究了冷喷涂涂层的微观结构，以探究其与腐蚀性能的关联。通过X射线三维计算机断层扫描技术观察了冷喷涂涂层的内部组织形貌，详细分析了其形貌特征，获得了孔隙空间分布和尺寸分布的定量数据，建立了冷喷涂涂层内部组织形貌可视化及孔隙特征参数测量方法。涂层含有的孔隙，或者涂层的致密性对于其耐腐蚀性能具有至关重要的作用。在对铝合金及陶瓷增强铝合金复合涂层腐蚀性能研究中发现，添加硬质颗粒是一种有效的提高涂层致密性从而提高耐腐蚀性能的手段。冷喷涂复合涂层与镁合金基体组成的电偶对在热循环中的失效发生于镁合金基体内部（深度大于100微米），而不是涂层与基体的界面。对镁合金冷喷涂涂层的研究得到了相似的结论，不同的是，相对于镁基板，镁合金涂层以及碳化硅增强镁合金涂层都表现出更高的腐蚀电位，因此在冷喷涂镁合金涂层与镁基板的电偶对中，涂层为阴极。同时陶瓷颗粒的添加提高了电偶电位，降低了涂层与镁基板的电偶电流，从而增强了涂层与基板间的电偶作用。本文研究为冷喷涂技术的应用，尤其是冷喷涂轻质合金涂层的应用及轻质合金零部件的冷喷涂修复提供了理论支撑。